This invention relates to display elements of the type in which the display element comprises a rotatable assembly including a reflective face and a permanent magnet acted upon by an electromagnet to drive the assembly between a first position in which the reflective face is exposed to the viewer and a second position in which it is hidden. Display devices of this type are described in U.S. Pat. Nos. 4,163,332; 3,975,728; 4,425,864; 4,015,255; and others.
With such display elements the energy needed to switch the element between its two visual states is dependent on the moment of inertia of the rotatable assembly. The rate of wear of the bearings of the rotatable assembly is also dependent on the moment of inertia. An object of the present invention is to reduce the moment of inertia and hence to reduce both the switching energy required and the rate of bearing wear, and thus to extend the life of the display element. The reflective face is the largest item of the rotatable display assembly, contributing highly to the moment of inertia. The present invention enables the reflective face to be made of materials which have little or no structural rigidity. Such materials are typically thin and light when compared with materials which have structural rigidity, and hence their use in this invention reduces the moment of inertia of the rotatable assembly considerably in comparison with prior art display assemblies.
The present invention provides an electromagnetically operated display element including a rotatable element comprising:
a support member rotatable about an axis and provided with bearing means;
mounting means connected to the rotatable support member;
a display sheet having opposite faces of contrasting appearance, the sheet including engagement means arranged to engage with the mounting means;
the rotatable support member, the mounting means and the sheet being arranged so that the sheet is permanently maintained in a substantially planar state.
According to another aspect this invention provides an electromagnetically-operated display element including a rotatable assembly comprising:
a support member rotatable about an axis, the support member including bearing means and having a permanent magnet attached to it; retaining means close to the axis connected with said support member;
arms connected with the support member;
a display sheet having opposite faces of contrasting appearance, the sheet including first engagement means arranged to engage with the retaining means and second engagement means remote from said axis arranged to engage with the arms
the support member said arms and said sheet being arranged so that the sheet is permanently maintained in a desired state with regard to the support member.
Preferably, in either of the above arrangements the sheet is thin and so has little or no structural rigidity and the mounting means applies a tensioning force to the sheet whereby to maintain it in a substantially planar form.
The invention will be better understood by reference to the attached drawings.